Shiny Red Balloon

It stretches your normal jump, both how long you keep rising and how quickly you leave the ground. Early on that is the difference between clearing a ledge and scrabbling at it. Under water it works against you and you jump slightly lower than you would with nothing equipped at all, so take it off before an Ocean dive.

The real value is what it does to double jumps. Every bottle accessory gets its second jump scaled up by the balloon, and the taller the base jump the bigger the gain, so pairing it with a Sandstorm in a Bottle produces a leap that clears roughly what low-tier wings would. Nearly all of those combinations are steps toward the Bundle of Balloons, so keep the balloon even after you stop wearing it raw. It also stacks with the Frog Leg, and it stretches Rocket Boots flight further per charge.

The Honey Balloon and the Balloon Pufferfish already contain this exact bonus, so wearing one alongside the plain balloon wastes a slot. The Tsunami in a Bottle is the odd one out of the double jumps: it cannot be tinkered with the red balloon and needs the pufferfish instead.